Enter the room left of the starting point and go to the back side of the block in the southwest. There is a blue chip on the block. Use it to raise another block so you can get the health bonus on the ledge. (sector 4)

Enter the room to the right of the starting point and climb the hills up. Face west and shoot at the ? block to lower the water in the room. This will allow you to enter a room with dead imps and an arch-vile (two on Hurt Me Plenty or higher) as well as the blue key and a berserk pack (not on Ultra-Violence or higher). (sector 119)

Open the blue door and enter the southeast corner, climbing the tiny ledges up until you reach a health bonus. (sector 263)

At the yellow key door, climb the hills to the top and head west to the stimpack. (sector 440)

From the platforms leading to the yellow key arena, jump southwest to the outer ridge and hug the waterfall until you drop in. You will find the red key at the bottom. (sector 296)

In the exit room, press the first switch and fight the enemies on the other side. Go over the exit switch and look towards the southwest for a red lock (you may have to wait a bit). Use the red key from Secret #5 to open a teleporter to the north. From here, you can get Secret #7 and the last health bonus needed for Secret #8. (sector 513)

In Secret #6, go up two of the grassy ledges and walk through the waterfall for a switch, which will reveal a megaarmor and a soul sphere in the exit room. (sector 525)

Once you have triggered Secrets #1, #3, and #6, return to the room left of the starting point and go to the northwest corner, falling down the waterfall to a large secret area, which will eventually allow you to reach a hidden area with a lot of monsters and a BFG9000 as well as an alternate exit. (sector 1)